Water-solid interfaces probed by high-resolution atomic force microscopy

Water-solid interfaces play important roles across a broad range of scientific and application fields. In the past decades, atomic force microscopy (AFM) has significantly deepened our understanding of water-solid interfaces at molecular scale. In this review, we describe the recent progresses on probing water-solid interfaces by noncontact AFM, highlighting the imaging of interfacial water with ultrahigh spatial resolution. In particular, the recent development of qPlus-based AFM with functionalized tips has made it possible to directly image the H-bonding skeleton of interfacial water under UHV environment. Based on high-order electrostatic forces, such a technique even enables submolecular-level imaging of weakly bonded water structures with negligible disturbance. In addition, the three-dimensional (3D) AFM using low-noise cantilever deflection sensors can achieve atomic resolution imaging at liquid/solid interfaces, which opens up the possibility of probing the hydration layer structures under realistic conditions. We then discuss the application of those AFM techniques to various interfacial water systems, including water clusters, ion hydrates, water chains, water monolayers/multilayers and bulk water/ice on different surfaces under UHV or ambient environments. Some important issues will be addressed, including the H-bonding topology, ice nucleation and growth, ion hydration and transport, dielectric properties of water, etc. In the end, we present an outlook on the directions of future AFM studies of water at interfaces and the challenges faced by this field, as well as the development of new AFM techniques.